Many optical systems utilize optical fibers to deliver light from a remote light source to a target destination. In a typical system, the light source is coupled to the fiber and light supplied by the source is guided by the fiber to the target destination. Optical fibers have been widely used in telecommunications to deliver information encoded in the form of an optical signal. A telecommunications link includes a transmitter that converts an electrical signal to an optical signal. The optical signal is launched into the fiber and transmitted to a receiver that reconverts the optical signal back to an electrical signal for further processing at the destination end of the link. Optical fibers have also been used as point illumination sources. In these applications, light from a source is coupled to the receiving end of the fiber and emerges from the destination end of the fiber as an illuminating beam.
There has recently been interest in extending the use of optical fibers to applications in broad-area illumination. In these systems, the objective is to achieve controlled release of light along at least portions of the length of the fiber to provide an illumination effect. Instead of using the fiber to confine light and transmit it with minimal losses to provide point illumination to a target positioned in the direction of the fiber axis (on-axis illumination), the objective is to use the lateral surface of the fiber as a broad-area source of illumination that operates in off-axis directions (azimuthal directions) of the fiber.
Light-diffusing fibers are a class of fibers that can be used as a broad-area illumination source. Light-diffusing fibers are designed to scatter light propagating along the fiber axis in azimuthal directions. The scattering is accomplished by incorporating nanostructural voids within or throughout the core and/or cladding regions of the fiber. The voids are low-index regions, typically filled with a gas, and have dimensions on the order of the wavelength of the light propagating through the fiber. The refractive index contrast between the voids and surrounding dense glass matrix effects scattering of the light. The scattering efficiency, and hence intensity of scattered light, can be controlled by controlling the dimensions, spatial arrangement and number density of voids. In addition to broad-area illumination, light-diffusing fibers can be employed in displays and as light sources in photochemical applications. Further information about light-diffusing fibers and representative applications can be found in U.S. Pat. No. 7,450,806 and U.S. Pat. Appl. Pub. No. 20110122646, the disclosures of which are hereby incorporated by reference herein.
Light-diffusing fibers are versatile and compact sources of broad-area lateral (off-axis) illumination and offer the further advantage of maintaining functionality when bent. This allows light-diffusing fibers to be deployed as illumination sources in tight spaces and areas where it is impossible to deploy conventional light sources.
To improve the aesthetic perception of illumination, it is desirable for the light-diffusing fiber to provide an illumination effect that is symmetric with respect to viewing angle. Symmetric illumination requires symmetric scattering of light from the light-diffusing fiber, which imposes significant practical constraints on the number, size, uniformity, and spatial distribution of voids in the light-diffusing fiber. To overcome the constraints, current light-diffusing fiber systems employ one of two designs. In a first design, two light sources positioned at opposite ends of the fiber are used to provide the illumination light. The light sources deliver light to the core portion of the light diffusing fiber and the core light is scattered to provide illumination. Any asymmetry in scattering of light propagating in one direction along the fiber is counteracted by light propagating in the opposite direction to provide a net illumination effect that is more nearly symmetric with respect to viewing angle. In a second design, a single light source is coupled to one end of the light-diffusing fiber and the opposite end of the light-diffusing fiber is coated with a reflective material. Source light that propagates through the fiber is reflected to provide counter-propagating light that improves the symmetry of illumination.
Applications of light-diffusing fibers as illumination sources could be expanded if systems capable of providing symmetric illumination that utilize a light-diffusing fiber with a single light source without a reflective end coating were available. Current systems that utilize a light-diffusing fiber with a single light source and no reflective end coating provide asymmetric illumination and inferior illumination aesthetics. There is a need to design light-diffusing fibers capable of providing symmetric illumination when illuminated with a single light source in the absence of counter-propagating light that corrects the symmetry of illumination with respect to viewing angle.